Recently, with the rapid development of the electric/electronic technologies, various electronic devices (e.g., video cameras, digital still cameras, minicams adapted to personal computers (PC), minicams adapted to mobile phones, etc.), employing image sensor technologies have been widely developed and used.
In the conventional devices as described above, a charge coupled device (CCD) has generally been used as the image sensor. However, the CCD has drawbacks. For example, the CCD requires high driving voltage, and separate additional support circuitry. Further, the CCD has high per-unit prices. As a result of these drawbacks, usage of CCDs has been on the decline recently.
Recently, Complementary Metal Oxide Semiconductor (CMOS) image sensors have attracted attention as an image sensor which overcomes the disadvantages of the CCD. Since the CMOS image sensor is manufactured based on a series of CMOS circuit technologies, contrary to the existing CCD, The CMOS image sensor is advantageous in that it can be driven with low voltage, it does not require additional support circuitry, it has a low per-unit price and so on.
For example, prior art CMOS image sensors are described in U.S. Pat. No. 6,583,484 entitled “Method of manufacturing photo diode CMOS image sensor”, U.S. Pat. No. 6,507,059 entitled “Structure of a CMOS image sensor”, U.S. Pat. No. 6,495,391 entitled “Invention for reducing dark current of CMOS image sensor with new structure”, U.S. Pat. No. 6,372,603 entitled “Photo diode with tightly-controlled junction profile for CMOS image sensor with STI process”, U.S. Pat. No. 6,350,127 entitled “Method of manufacturing for CMOS image sensor”, and Japanese Patent No. 2000-31525 entitled “Photo diode of image sensor and manufacturing method thereof.”
As shown in FIG. 1, a conventional image sensor, (e.g., a CMOS image sensor), includes a photo diode 3 formed on an active region of a semiconductor substrate 1 defined by an element isolating layer 2 so as to generate and accumulate a series of photocharges through external incident light. It also includes a signal process transistor 10 disposed near the photo diode 3 so as to carry/discharge the photocharges generated and accumulated by the photo diode 3 toward an image processing circuit (not shown). In the illustrated example, the signal process transistor 10 includes, for example, a gate insulating layer pattern 11, a gate electrode pattern 12, a spacer 13 and an impurity layer 14.
In the example shown in the drawings, N type impurities (which are indicated as N+ in the drawing) of an impurity layer 3a for forming the photo diode 3 are relatively heavily doped compared with a P type semiconductor substrate 1. Thus, a depletion region (DR) formed upon the operation of the sensor is induced to extend inside the semiconductor substrate 1.
The reason why the DR is induced to extend inside the substrate 1 through the relatively heavy doping of the N type impurities compared with the P type semiconductor substrate 1 is as follows. If the N type impurities are relatively lightly doped compared with the P type semiconductor substrate 1 without a separate measure so that the DR formed upon the operation of the sensor is induced to extend toward the surface of the semiconductor substrate 1, various defects widely existing around a surface 1a of the semiconductor substrate (i.e., the surface of the photo diode) have bad effects upon the DR without a particular limit. Therefore, a finished photo diode operating in this fashion may exhibit various defective phenomena such as noise, dark current and so on.
In contrast, if the N type impurities are relatively heavily doped compared with the P type semiconductor substrate to induce the DR formed upon the operation of the sensor to extend inside the semiconductor substrate 1 as described above, the photo diode 3 must face the problem that light which cannot deeply penetrate the semiconductor substrate due to its short wavelength, (e.g., blue light), cannot normally be received in the DR. As a result, the finished image sensor has a greatly reduced ability to reproduce colors of short wavelength (e.g., blue) light.
Meanwhile, as shown in the drawing, a silicide layer 15 is additionally formed on a part of the signal process transistor 10. For example, the silicide layer 15 may be formed on the surfaces of the gate electrode pattern 12 and the impurity region 14 so as to improve a contact quality thereof.
However, if separate measures are not taken, the various defects caused by the formation of the corresponding silicide layer 15 are widely generated on the surface la of the semiconductor substrate 1. That is, if the formation process for the silicide layer 15 is forcedly performed in the state where the surface 1 a of the semiconductor substrate 1 is exposed without any separate covering means, damage will widely occur on the surface 1a of the semiconductor substrate 1 by various processing shocks applied during the formation process for the silicide layer 15.
In order to resolve such problems, in the prior art as shown in the drawing, a blocking layer 4 is previously formed on the surface 1a of the semiconductor substrate 1 so as to prevent damage on the surface 1a of the semiconductor substrate 1 caused by the formation of the silicide layer 15 through a function of the blocking layer 4. However, while damage to the semiconductor substrate 1 caused by the formation of the silicide layer 15 may be prevented in some degree by the blocking layer 4, other damage caused by the formation of the blocking layer 4 will necessarily be generated on the surface 1a of the semiconductor substrate 1, which greatly degrades a characteristic of the photo diode.
Furthermore, image sensor manufacturers face not only the degradation of the characteristic of the photo diode 3 due to the formation of the blocking layer 4, but also the reduction in the overall process efficiency inherent in the inclusion of an additional step to form the blocking layer 4.
In the following description and drawings, the same reference numerals are used to designate the same or similar components, and so repetition of the description of the same or similar components will be omitted.